1. Field of the Invention
This invention relates to a system for estimating the speed of a body remotely. The invention is particularly applicable to the estimation of speed of a mobile unit in a mobile communications, e.g. cellular radio, system.
2. Related Art
Methods of effecting handover of a call to or from a mobile unit between cells in a cellular radio telephone system are very important to the overall efficiency and quality of the service provided. The object is to achieve a fast and reliable handover between cells.
In particular, busy areas having a high call density can comprise a large macrocell, covering a number of streets extending for one or two kilometers in all directions, and a number of overlaid microcells which are located in streets where a particularly heavy call density is expected. These microcells are typically about 200 meters long and may extend simply along a straight street.
The handover between a microcell and a macrocell is subject to special considerations. It is important for the system to be able to determine whether it is worthwhile effecting a handover of a mobile unit from a macrocell to a microcell as the latter is approached. It may be that the unit is going to enter the microcell for a significant period or only relatively momentarily as the microcell is crossed.
An important factor is the determination of the mobile unit speed. If the mobile unit is stationary and experiences bad reception, only intracell handover is required. An assignment of a different channel with less interference or fading within the same base station can easily improve the quality of the link. By contrast, if the mobile unit is moving, a decision has to be made on whether an intercell or an intracell handover has to be initiated. Furthermore, as handportable mobile telephones are anticipated to be more widely used in future, they may move slowly or stay within a locality for a long time. However, there is also a chance that the handportable mobile unit is being used on a vehicle.
The assessment of vehicle speed can be carried out simply by measuring directly through, for example, the transmission of a motor vehicle. However this is unsuited to cellular radio applications.
As an alternative, the vehicle speed may be estimated by monitoring the Doppler shift of a carrier frequency. This method requires a stable frequency standard which is expensive and also impracticable if the frequency generator is to be included in each hand portable unit.
Another alternative is to use the elapsed time between two or more successful handovers. A user moving at walking pace would have a significantly longer elapsed time between handovers than when the person is travelling in a vehicle. This method, while feasible when operating on its own, has the disadvantage of requiring a relatively long training time delay of at least two or more handovers. It cannot adapt to sudden changes of speed, such as when a user alights or boards a vehicle in between handovers. Furthermore, it cannot be used immediately after the user joins the system. Consequently, it can only be used as a backup speed estimator.